Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
SW1# show spanning-tree vlan 10
VLAN0010
Root ID Priority 32778
Address 0011.2233.4456
Bridge ID Priority 32778
Address 0011.2233.4456
SW2# show spanning-tree vlan 10
VLAN0010
Root ID Priority 32778
Address 0011.2233.4455
Bridge ID Priority 32778
Address 0011.2233.4455Based on the exhibit, why did SW2 become the root bridge for VLAN 10?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Best answer
Because SW2 has the lower bridge ID due to the lower MAC address.
This is correct because the priorities are equal, so the lower MAC address wins the root election.
B
Distractor review
Because SW2 has the higher VLAN number configured.
This is wrong because VLAN number does not decide the root bridge election.
C
Distractor review
Because SW2 has more trunk ports than SW1.
This is wrong because port count does not determine root bridge election.
D
Distractor review
Because SW2 has the highest bridge priority.
This is wrong because lower, not higher, bridge ID wins.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is to incorrectly believe that the root bridge election depends on the VLAN number, the number of trunk ports, or the highest bridge priority. Candidates often confuse the priority value with the bridge ID or think that more trunk ports or higher VLAN numbers give a switch an advantage. In reality, STP root bridge election strictly compares bridge IDs, which combine priority and MAC address. If priorities are equal, the switch with the lower MAC address wins. Misunderstanding this can lead to incorrect answers and misinterpretation of STP behavior in VLAN environments.
Technical deep dive
How to think about this question
Spanning Tree Protocol (STP) is a Layer 2 network protocol that prevents switching loops by electing a single root bridge for each VLAN. The root bridge acts as the logical center of the spanning tree topology. Each switch in the network has a unique Bridge ID (BID), which is composed of a configurable priority value and the switch’s MAC address. STP uses these BIDs to elect the root bridge, with the lowest BID winning the election. The root bridge election process compares the BIDs of all switches participating in the VLAN. The priority value is the primary factor; if multiple switches share the same priority, the MAC address serves as the tiebreaker. Since MAC addresses are unique and fixed, the switch with the lowest MAC address wins if priorities are equal. This ensures a deterministic and stable root bridge selection. In the exhibit, SW2 became the root bridge because it had the same priority as SW1 but a lower MAC address. A common exam trap is to assume that the root bridge is always the switch with the lowest priority or the highest VLAN number, or that port count or other factors influence the election. However, STP strictly follows the BID comparison rule, prioritizing lower priority first and then lower MAC address. Practically, this means network engineers can influence root bridge selection by adjusting priority values, but if priorities are equal, the MAC address determines the root. Understanding this helps avoid misconfigurations and ensures predictable STP behavior in Cisco networks.
KKey Concepts to Remember
- STP elects the root bridge by comparing bridge IDs, which consist of a configurable priority and the switch’s MAC address.
- The switch with the lowest bridge ID becomes the root bridge for a given VLAN in the spanning tree topology.
- If multiple switches have the same priority, STP uses the lowest MAC address as the tiebreaker to elect the root bridge.
- VLAN numbers do not influence the root bridge election process in STP.
- The number of trunk ports on a switch does not affect the root bridge election decision.
- A higher bridge priority value does not help a switch become the root; lower priority values are preferred.
- Understanding the bridge ID composition is essential for predicting and controlling STP root bridge election outcomes.
- STP root bridge election is deterministic and prevents loops by ensuring a single root per VLAN.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
STP elects the root bridge by comparing bridge IDs, which consist of a configurable priority and the switch’s MAC address.
What is the correct answer to this question?
The correct answer is: Because SW2 has the lower bridge ID due to the lower MAC address. — SW2 became the root bridge because its bridge ID is lower. In practical terms, spanning tree elects the root bridge by comparing bridge IDs, which are based on priority plus MAC address. The device with the lowest bridge ID wins. In the exhibit, both switches use the same priority, so the tie is broken by the lower MAC address. This is a classic STP interpretation question. Many learners focus only on priority, but if priorities match, the MAC address becomes decisive.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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